1. Field of the Invention
This invention relates to a track jump control circuit in which a scanning head for scanning the recording tracks of a disc-shaped recording medium is moved to a target track position. This invention may be applied, for example, for controlling the scanning of an optical disc device in which a recording track formed on the optical disc in the form of a circumambient pattern is scanned by an optical beam produced by an optical head for recording and/or reproducing information signals along the recording track.
2. Prior Art
Known optical discs include an overwritable optical disc, such as a magneto-optical disc, a write-once type optical disc or unrewritable ROM type optical disc.
In such an optical disc device employing these optical discs as recording media, focusing servo or tracking servo is applied to an optical head having accommodated therein a laser diode for producing a data recording and/or reproducing laser beam or a photodetector for detecting the reflected laser light from the optical disc, whilst the optical disc is rotated at a constant angular velocity or at a constant linear velocity. The recording track formed in a circumambient pattern on the optical disc is scanned in this manner by the laser light for performing data recording or reproduction.
In such an optical disc apparatus in which a recording track or tracks formed on an optical disc in a circumambient pattern is scanned by a light beam from an optical head for recording and/or reproducing signals along said recording track, it is necessary to perform tracking control so that the recording track will be correctly scanned by the spot of the light beam scanning the recording track. To this end, a tracking error signal corresponding to the deviation of the light spot from the recording track in the track pitch direction is derived from the output of the optical head and fed back to a tracking actuator driving circuit for bringing the center of the light beam spot into coincidence with the track center through tracking control. A three-beam method and a push-pull method are known for detecting tracking errors in the optical disc apparatus.
In conventional optical disc apparatus, when the beam spot traverses the recording track, a tracking error signal S.sub.TE which changes sinusoidally as indicated by the formula ##EQU1## wherein x denotes a displacement of the beam spot from a track center of a recording track TK.sub.(n) in the track pitch direction and p denotes the track pitch, is derived from a detection output from the optical head, as shown in FIG. 1, and tracking control is performed so that the tracking error signal S.sub.TE will be reduced to zero.
On the other hand, for a track jumping operation, that is, for shifting the beam spot to an adjacent track or to a track spaced by several tracks from the current track, the tracking control is turned off and the optical head is moved at an elevated speed in a controlled manner so that the target speed will be reduced to approximately zero at the target track position, and the tracking control is turned on as soon as the target track position is reached so that the beam spot lands on the target track.
As one of the techniques for realizing a unified recording format for the above-mentioned various optical discs, a so-called sampled servo technique is proposed, according to which, similarly to the so-called sector servo in a hard disc (one of the magnetic disc types) servo signals are recorded (that is, preformatted) by clock pits or tracking pits at a predetermined distance or angular interval on the concentrically or spirally extending track or tracks on the disc, these discrete servo signals being sample-held during disc rotation to effect continuous servo control. Such a known optical disc is shown in FIG. 2 as an optical disc 10 having the recording format illustrated therein.
Referring to FIG. 2, the optical disc 10 has an annular label section 2 around a central opening 1 and an annular recording surface section 3 extending around the label section 2. In the recording surface section 3 is formed a spirally extending recording track tk in a circumambient pattern about the central opening 1. Each turn or circumambient track is divided into a predetermined number m of sectors sc.sub.1, sc.sub.2, . . . , sc.sub.m. Herein, m=32. The sectors bearing the same postscript number in the respective circumambient tracks, such as sc.sub.1, are arrayed in the same radial direction of the optical disc 10. The optical disc 10, provided with these recording tracks tk, is loaded into an optical disc recording/reproducing apparatus, and is rotated in the direction shown by an arrow r so as to be used for data recording and/or reproduction with the aid of an optical beam.
Each sector sc.sub.1, sc.sub.2, . . . or sc.sub.m in each circumambient track is constituted by an address data section ad at the leading end and a predetermined number n such as 43 of blocks bl.sub.1, bl.sub.2, . . . , bl.sub.n, arrayed next to the address data section ad along the recording track tk. These blocks bl.sub.1, bl.sub.2, . . . , bl.sub.n are so arrayed that the blocks bearing the same postscript, such as blocks bl.sub.1, of the sectors sc.sub.1, sc.sub.2, . . . , sc.sub.m are arrayed in the radial direction of the optical disc 10. Each of these blocks bl.sub.1, bl.sub.2, . . . , bl.sub.n of the sectors sc.sub.1, sc.sub.2, . . . , sc.sub.m has a control record region ar.sub.C at the start end, followed by a data record region ar.sub.D to constitute a unit recording section. In the control record region ar.sub.C of each block bl.sub. 1, bl.sub.2, . . . or bl.sub.n, tracking data pits q.sub.a and q.sub.b are formed on the outer and inner sides of a track centerline kc and a clock data pit q.sub.c on the track centerline kc. The manner in which the track data pits q.sub.a and q.sub.b and the block data pit q.sub.c are arrayed in a direction normal to the track centerline kc, that is, along the radius of the optical disc 10, is hereinafter explained. Referring to FIG. 3, the tracking data pits q.sub.b and the clock data pits q.sub.c are arrayed each in one line along a radius, whereas the tracking data pits q.sub.a are offset at intervals of 16 consecutive tracks in the longitudinal direction of the track tk. When the optical disc 10, in which the tracking data pits q.sub.a and q.sub.b and the clock data pits q.sub.c are provided in this manner in the control record region ar.sub.c, is loaded in the recording/reproducing apparatus so as to be used for data recording or reproduction with the aid of an optical beam, the tracking data pits q.sub.a and q.sub.b and the clock data pits q.sub.c of the control record region ar.sub.c are read by the light beam so as to be used for various servo operations or clock generation. That is, clock signals are detected from the detected output of the clock data pits q.sub.c to generate necessary timing clocks, while the tracking error is found on the basis of the detected output of the tracking data pits q.sub.a and q.sub.b arrayed on the outer and the inner sides of the track centerline kc to perform tracking control. On the other hand, focusing control is performed on the basis of the detected output of a mirror surface region. The detected output of the tracking data pits q.sub.a is also used for performing a so-called traverse counting to find the number of the track being scanned by the optical head on the basis of the above mentioned offset of the tracking data pits q.sub.a at intervals of 16 consecutive tracks.
Meanwhile, an optical disc usually suffers from offsets, so that the target track position deviates as a result of the disc's rotation. Thus the speed of movement of the optical head which is detected by a speed sensor provided on the side of a mechanical base fails to indicate the true speed with which the light spot is moving on the optical disc.
Recently, an attempt has been made to detect the speed with which the light beam spot travels on the optical disc.
However, in the sampled servo system, the track traversing speed cannot be detected unless the light spot traverses one track within a time at least twice the sampling period, as may be demonstrated by the sampling theorem, such that the speed during high speed movement cannot be detected.
On the other hand, in the optical disc device of the sampled servo system, the tracking error signal S.sub.TE is produced on the basis of the detected output of the tracking data pits q.sub.a and q.sub.b, and a track jump driving signal S.sub.JP formed from the tracking error signal S.sub.TE, as shown in FIG. 4. The track jump driving signal S.sub.JP is supplied in an open loop to a tracking actuator to perform track jump control by a feedforward operation.
In the above described optical disc device utilizing the sampled servo system, the tracking error signal S.sub.TE is produced on the basis of the detected output of the tracking data pits q.sub.a and q.sub.b. The track jump driving signal S.sub.JP is formed from the tracking error signal and supplied in an open loop to a tracking actuator to actuate the track jump drive by a feedforward control operation. Accordingly, the disc device is susceptible to malfunction due to disturbances and, above all, by vibrations, and hence a stable track jump operation cannot be performed in an environment likely to undergo excessive vibrations.
On the other hand, when the optical head is moved to the target track position to perform data recording and/or reproduction on or from the target recording track, the accessing speed cannot be detected with high accuracy and hence the target track cannot be reached by one accessing operation resulting in a prolonged access time.
When the track jump operation is performed with the use of gray code data, supposing that the gray code data is recorded at an interval of 12 .mu.s on a recording track having a track pitch of, for example, 1.6 .mu.s, speed data can be detected only up to 0.13 .mu.m/s, on account of the properties of the gray code data. In addition, if the servo is range of the tracking servo equal to 3 kHz, for example, track capturing becomes infeasible at an inrush speed at or above 8 mm/s, with the result that direct landing on the target track cannot be achieved.